N. Prakashet al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Research Paper
POWER QUALITY IMPROVEMENT OF GRID INTER
CONNECTED HYBRID SYSTEM USING STATCOM
1
N. Prakash, 2V R. Balaji, 3M. Sudha
Address for Correspondence
1,2,3
Department of Electrical and Electronics Engineering, Kumaraguru College of Technology, Coimbatore, India.
ABSTRACT
A Power quality problem is an occurrence deals as a nonstandard voltage, current or frequency that results in a failure or a
mis-operated in end user equipment. Utility distribution networks, sensitive industrial loads and critical commercial operations suffer from various types of outages and service interruptions which can cost significant financial losses. With the restructuring of power systems and with shifting trend towards distributed and dispersed generation, the issue of power quality
is going to take newer dimensions. Injection of the wind power into an electric grid affects the power quality. The performance of the wind turbine and thereby power quality are determined on the basis of measurements and the norms followed
according to the guideline specified in International Electro-technical Commission standard, IEC-61400. The influence of
the hybrid energy in the grid system concerning the power quality measurements are active power, reactive power, harmonics, and electrical behaviour of switching operation and these are measured according to national/international guidelines.
The paper study demonstrates the power quality problem due to installation of wind turbine and photovoltaic system with the
grid. In this proposed scheme STATIC COMPENSATOR (STATCOM) is connected at a point of common coupling with a
battery energy storage system (BESS) to mitigate the power quality issues. The battery energy storage is integrated to sustain
the real power source under fluctuating wind power. The STATCOM control scheme for the grid connected hybrid energy
generation system for power quality improvement is simulated using MATLAB/SIMULINK in power system block set.
Finally the proposed scheme is applied for both balanced and unbalanced non linear loads.
KEYWORDS Power Quality, Wind Generating System (WGS), STATCOM, BESS, International electro-technical commission (IEC) standard.
1. INTRODUCTION
Energy plays an important role in our daily life activities. As there is a large increase in population, urbanization and industrialization, there is increase in
energy demand too that is increasing day by day. The
major fossil fuels like coal, petroleum and natural gas
are depleting day by day. Moreover it is assumed that
these fossil fuels will be depleted in few hundred
years. The phenomenon of increasing the rate of energy consumption and supply is decreasing that result
into energy shortage. This is referred to as energy
crisis. So as to meet future energy requirement, any
other alternative or renewable sources of energy have
to be developed. All non conventional energy sources
have geographical limitations except solar energy.
Solar energy has less geographical limitations as
compared to other non conventional sources of energy because solar energy is available over the entire
globe. Nowadays solar energy has become one of the
most promising renewable energy due to its inexhaustible and environmental advantages. Solar energy is set to play an ever increasing role in generating
the form, affecting the appearance and construction
of buildings. The principal reason for this is that
photovoltaic systems which produce electricity directly from solar radiation are becoming more widespread as their advantages become apparent and as
costs fall. The influence of the wind turbine in the
grid system concerning the power quality measurements are-the active power, reactive power, variation
of voltage, harmonics, and electrical behavior of
switching operation.This paper study demonstrates
the power quality problem due to installation of wind
turbine with the grid.
Power quality-The issue
Ideally, the goal of power industry is to supply a
purely sinusoidal voltage at fixed amplitude and
fixed frequency. Whereas it is the duty of the supplier
to provide an almost sinusoidal voltage with less
variation in amplitude and frequency, the user also
has a part to play in creating such a scenario. The
interface point at which the utility company’s responsibility ends and the user’s responsibility starts is
often termed as the point of common coupling (PCC).
At the PCC, both the utility company and the user
have some factors to comply with. While the utility
company has to provide reliable power, the user has
to ensure that the load connected does not lead to
higher losses in the generation, transmission and distribution systems. Considering some arbitrary load,
the total current consumed can be split into three
components – active, reactive and harmonic. Whereas the active current flow leads to real power consumption and is subsequently responsible for the energy absorbed by the system to do work, the reactive
and harmonic currents do not lead to any net energy
transfer. While the reactive current is required to establish the magnetic medium and is responsible for
the energy conversion in electrical systems, the harmonic currents are the result of the switching devices
used in electronic and power electronic systems. So,
it is evident that one cannot do away with the reactive
and harmonic currents. However, since the net energy
transfer due to them (during any given fundamental
period) is zero, it is not required that these currents
have to be taken from the grid. It must be noted that,
the ‘zero energy transfer’ we are talking about is at
the PCC. That is, losses at the generation, transmission and distribution systems do exist when loads
draw reactive and harmonic currents. In order to
bring down these losses, the utility companies require
the user to absorb a nearly purely active current.
Power quality standards, Issues and its consequences
International Electro Technical Commission
Guidelines
The guidelines are provided for measurement of
power quality of wind turbine. The standard norms
are specified [5].
1) IEC 61400-21: Wind turbine generating system,
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N. Prakash et al., International Journal of Advanced Engineering Technology
part 21. Measurement and Assessment of power
quality characteristic of grid connected wind turbine.
2) IEC 61400-13: Wind turbine measuring procedure
in determining the power behavior.
3) IEC 61400-3-7: Assessment of emission limit for
fluctuating load.
Voltage Variation
The voltage variation issue results from the wind
velocity and generator torque. The voltage variation
is directly related to real and reactive power variations. The voltage flicker issue describes dynamic
variations in the network caused by wind turbine or
by varying loads [4].
The voltage variation is commonly classified as under:

Voltage Sag/Voltage Dips.

Voltage Swells.

Short Interruptions.

Long duration voltage variation.
Harmonics
The harmonic results due to the operation of power
electronic converters. The harmonic voltage and current should be limited to the acceptable level at the
point of wind turbine connection to the network.
Consequences of the issues
The voltage variation, harmonics and reactive power
causes the mal-function of equipments like controllers. It may leads to tripping of contractors, tripping
of protection devices, stoppage of sensitive equipments like personal computer, programmable logic
control system and may stop the process and even
can damage of sensitive equipments. Thus it degrades
the power quality in the grid.
2. WIND ENERGY CONVERSION SYSTEM
The wind energy conversion consists of a wind turbine connected to a doubly fed induction generator
(DFIG).
The DFIG will produce power depends
on the speed of wind. The major problem with wind
power generation is uneven wind speed. Another
main problem in wind energy generation is the connection to the grid. Injection of wind power into the
grid affects the power quality resulting in poor performance of the system. [6]The wind energy system
faces frequently fluctuating voltage due to the nature
of wind and introduction of harmonics into the system.
Design of doubly fed induction generator
The Doubly Fed Induction Generator (DFIG) based
wind turbine with variable-speed variable-pitch control scheme is the most popular wind power generator
in the wind power industry. This machine can be operated either in grid connected or standalone mode. A
thorough understanding of the modeling, control, and
dynamic as well as the steady state analysis of this
machine in both operation modes is necessary to optimally extract the power from the wind and accurately predict its performance. In this project a detailed electromechanical model of a DFIG-based
wind turbine connected to power grid as well as autonomously operated wind turbine system with integrated battery energy storage is developed in the
MATLAB/Simulink environment and its corre-
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sponding generator and turbine control structure is
implemented. Typically, most of the wind turbines
are located at remote places or offshore where the
power grid is usually long and weak characterized by
under voltage condition. Because of the limited reactive power capability, DFIG cannot always supply
required reactive power; as a result, its terminal
voltage fluctuates. Hence, a voltage regulation device
is required for the secure operation of the overall
wind turbine together with power grid during normal
operation as well as disturbances in the grid. Flexible
ac transmission system (FACTS) devices, through
their fast, flexible, and effective control capability,
provide solution to this challenge. Therefore, this
thesis examines the use of Static Synchronous Compensator (STATCOM) at the Point of Common Coupling (PCC) to regulate terminal voltage of the DFIG
wind turbine system. The series compensation in the
transmission line to improve steady state voltage and
enhance power carrying capability of the line is also
examined. Simulation results verify the effectiveness
of the implemented system for steady state as well as
dynamic voltage regulation.
Real and Reactive Power Capability of the DFIG
With the increased penetration level of wind power in
the power system, grid utilities want wind turbine
generator system to behave like a conventional synchronous generator in terms of real and reactive
power settings [8]. In other words, wind turbines
have to contribute not only to active power generation but also to the reactive power. Hence, wind turbines should have extended reactive power capability
not only during voltage dips but also in steady state
operation. Although, the DFIG wind turbines are able
to control active and reactive power independently of
one another by virtue of ac/dc/ac power electronic
converter present on it, the reactive power capability
of those generators depend on the active power generated, the slip and the limitation due to 20. Following design parameters: 1) rotor voltage, 2) stator current and 3) rotor current. The grid side inverter reactive power capability can be taken into consideration,
but in commercial system, this converter usually
works with unity power factor, i.e. zero reactive
power, so the total reactive power capability of the
generator is equal to the stator reactive power capability. Therefore, in this thesis, the steady state operation of the DFIG wind turbine system is described
clearly through the characteristic curves. The steady
state active power flow in the stator and rotor side is
presented
for
sub-synchronous
and
super-synchronous operation modes of the DFIG. The
reactive power capability of the DFIG is studied
through the P-Q diagram. The reactive power capability is obtained for maximum power point operation
mode and is extended to the pitch control operation
mode of the DFIG as well which is not found in the
literature. The reactive power capability curve of the
overall DFIG with the STATCOM connected at the
PCC is derived to meet the steady state power factor
requirement.
Model of DFIG
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The DFIG consists of stator winding and the rotor
winding equipped with slip rings. The stator is provided with three-phase insulated windings making up
a desired pole design and is connected to the grid
through a three-phase transformer. Similar to the stator, the rotor is also constructed of three-phase insulated windings. The rotor windings are connected to
an external stationary circuit via a set of slip rings
and brushes. By means of these components, the
controlled rotor current can be either injected to or
absorbed from the rotor windings [12].
3. PROPOSED SYSTEM
PV Cell
The PV cells are made of semiconductor materials,
such as silicon. For solar cells, a thin semiconductor
wafer is specially treated to form an electric field,
positive on one side and negative on the other. When
light energy strikes the solar cell, electrons are
knocked loose from the atoms in the semiconductor
material. If electrical conductors are attached to the
positive and negative sides, forming an electrical
circuit, the electrons can be captured in the form of
an electric current - that is, electricity. This electricity
can then be used to power a load.
PV Cell Design
The standard one diode or 5 parameter model used to
represent the SPV module is shown in Figure 3.1. It
consists of a current source in parallel with a diode, a
shunt resistance and a series resistance. The modeling
equations are:
=
( , )=
=
−
=
=
+
Figure 3.1 Equivalent Circuit of a single PV Cell
Boost converter
The Boost converter is used to extract power from
the PV Array, in order to maintain the Constant
Power Output. In the Proposed model, the boost
converter boosts DC voltage from 273.5 V to 500V.
This converter uses a MPPT system which automatically varies the duty cycle in order to generate the
required voltage to extract maximum power.
Maximum Power point Tracking (MPPT) Algorithm: (Incremental Conductance Method)
In this proposed System, Incremental Conductance
(INC) algorithm is used for MPPT. The incremental
conductance uses the PV array's incremental conductance dI/dV to compute the sign of dP/dV. It does
this using an expression derived from the condition
that, at the maximum power point, dP/dV = 0. Be-
ginning with this condition, it is possible to show
that, at the MPP dI/dV = -I/V. Thus, incremental
conductance can determine that the MPPT has
reached the MPP and stop perturbing the operating
point. If this condition is not met, the direction in
which the MPPT operating point must be perturbed
can be calculated using the relationship between
dI/dV and -I/V. The Incremental Conductance method can be mathematically derived as follows
( )
=
=
+
At MPP,
=0
V
dI
dI
+I=0
=−
Figure 3.2 Standard MPP Curve using INC Conductance Algorithm
Three phase Voltage Source Inverter (VSI) using
sinusoidal pulse width modulation (SPWM) technique
Single-phase VSIs cover low-range power applications and three-phase VSIs cover the medium- to
high-power applications. The main purpose of these
topologies is to provide a three-phase voltage source,
where the amplitude, phase, and frequency of the
voltages should always be controllable. Although
most of the applications require sinusoidal voltage
waveforms arbitrary voltages are also required in
some emerging applications (e.g., active filters, voltage compensators). As in single-phase VSIs, the
switches of any leg of the inverter (S1 and S4, S3 and
S6, or S5 and S2) cannot be switched on simultaneously because this would result in a short circuit
across the dc link voltage supply. Similarly, in order
to avoid undefined states in the VSI, and thus undefined ac output line voltages, the switches of any leg
of the inverter cannot be switched off simultaneously
as this will result in voltages that will depend upon
the respective line current polarity. Of the eight valid
states, two of them produce zero ac line voltages. In
this case, the ac line currents In order to generate a
given voltage waveform, the inverter moves from one
state to another. Thus the resulting ac output line
voltages consist of discrete values of voltages that are
vi , 0, and ÿvi .The selection of the states in order to
generate the given waveform is done by the modulating technique that should ensure the use of only
the valid states.
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Now, we define,V = V + V , where ‘j’ is the imaginary operator. ‘V’ is called the space vector. The
concept is similar to and has many advantages like
“phasor” representation of sinusoidal quantities.
V=V +V
3
3
V = V sin
− j V cos
2
2
3
3
V = −j V cos
+ j V sin
2
2
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3
( )
V
2
°
3
V= Vm ej ωt-90
2
The usefulness of ‘V’ is that it gives a good insight
about how the three phase quantities appear in the α-β
plane.
V=
Flow Chart for Incremental Conductance Algorithm
Figure 3.2.Flow Chart for Incremental Conductance Algorithm
STATCOM
The STATCOM (or SSC) is a shunt-connected reactive-power compensation device that is capable of
generating and or absorbing reactive power and the
output can be varied to control the specific parameters
of an electric power system. [7] Specifically, the
STATCOM considered in this chapter is a voltage-source converter that, from a given input of dc
voltage, produces a set of 3-phase ac-output voltages,
each in phase with and coupled to the corresponding
ac system voltage through a relatively small reactance
(provided by either an interface reactor or the leakage
inductance of a coupling transformer). The dc voltage
is provided by an energy-storage capacitor[10].
A STATCOM can improve power-system performance in such areas as the following:
1. The dynamic voltage control in transmission
and distribution systems
2. The power-oscillation damping in power
transmission systems
3. The transient stability
4. The voltage flicker control and
5. The control of not only reactive power but
also (if needed) active power in the connected line, requiring a dc energy source.
6. Furthermore, a STATCOM does the following:
It occupies a small footprint, for it replaces passive
banks of circuit elements by compact electronic con-
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verters. It offers modular, factory-built equipment,
thereby reducing site work and commissioning time
and it uses encapsulated electronic converters, thereby
minimizing its environmental impact. A STATCOM
is analogous to an ideal synchronous machine, which
generates a balanced set of three sinusoidal voltages at
the fundamental frequency with controllable amplitude and phase angle. This ideal machine has no inertia, is practically instantaneous, does not significantly
alter the existing system impedance, and can internally generate reactive (both capacitive and inductive)
power [3].
Figure.3.3 Basic STATCOM connection to a Grid
The STATCOM is in principle a voltage source converter (VSC) connected via an inductance to a grid.
The concept has been known for many years and is
described in detail figure 3.3 shows an example of a
STATCOM connected to a grid.
4.SIMULATION RESULTS
Grid interconnected hybrid system
S NO.
1
2
3
4
5
6
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The Grid interconnected Hybrid system consists of a
wind Energy Conversion System and Photo Voltaic
Energy Conversion System Connected to a electrical
utility grid. The WECS is comprised of a doubly fed
Induction Generator (DFIG) with self supplying reactive power [9]. The PV System consists of PV Array Supported with a Boost Converter. The grid interconnection converts the variable frequency and
magnitude outputs from the hybrid wind/PV system
to the synchronous frequency of the utility grid. The
variable frequency and magnitude output voltages
from the hybrid wind/SPV system are converted to
DC voltages or so called DC links.
The grid side inverter converts the DC link voltages
to the synchronous voltages of the grid. A Sinusoidal
Pulse Width Modulation (SPWM) is employed for
generating gate pulse. The power issues such as active power, reactive power and harmonics arouse
during interconnection of WECS and PV System to
the Electrical grid can be overcome with insertion of
STATCOM at the point of common coupling [1].
The Electrical grid is supplying to a non linear
load.The ultimate aim is to reduce the harmonics and
maintain the reactive power at a desired level. Thus
the power system can able to supply active power
efficiently to the load. [11] A hybrid energy system
usually consists of two or more renewable energy
sources used together to provide increased system
efficiency as well as greater balance in energy supply. In this project, the hybrid energy system is a
photovoltaic array coupled with a wind turbine. Figure 4.1 shows the schematic diagram of proposed
system.
TABLE 4.1 GRID INTERCONNECTED HYBRID SYSTEM DETAILS
PARAMETERS
RATINGS
Grid Voltage
3-Phase, 25kV, 50 Hz
Induction Motor/Generator
3.3 kVA,575 V,50Hz,P=4, Speed=1440 rpm, Rs= 0.01Ω, Rr=0.015
Ω,Ls=0.06 H, Lr=0.06 H
Photo Voltaic Energy Con- PV Array (66*5*305.2W=100.7 kW)
version System
Open-circuit voltage: Voc= 64.2 V
Short-circuit current: I sc = 5.96 A
Voltage and current at maximum power:
Vmp =54.7 V, Imp= 5.58 A
Line Series inductance
0.05mH
Inverter Parameters
DC Link Voltage=800V, DC Link Capacitance=100µF,Switching
Frequency= 2 kHz
IGBT Ratings
Collector Voltage = 1200V, Forward Current = 50A, Gate Voltage
=20V,Power Dissipation = 310W
Simulink model of hybrid energy conversion system
The MATLAB/ Simulink model Consists of 6 wind
turbines of 1.5 MW with 9 MW total Capacity. The six
wind turbine is paralleled with Doubly Fed Induction
Generator which is supplied with AC-AC Converter
with DC link at rotor side to provide self supporting
reactive power to rotor. This setup will provide a
WECS for reliable active power delivery to the electrical grid. The AC Voltage of 575V from WECS is
synchronized to 25kV with the help of a Potential
Transformer. The PV Energy System with Boost
Converter was designed and output DC Voltage is
inverted to AC Power using Single level VSI. The AC
Output Power of about 75kW with 260V AC Voltage
is synchronized to 25kW with a Potential Transformer
[2].
THD analysis with and without STATCOM for
hybrid system
THDAnalysis without STATCOM
The simulation result shows the output voltage and
current waveforms of the electrical grid without using
STATCOM. The voltage and current waveforms gets
distorted due to real and reactive power imbalance of
grid. The THD can be computed using FFT analysis.
The computation results in figure 4.2. In figure 4.3
shows that the voltage THD is about 47.86% of fundamental frequency and THD for current is about
12.22% of fundamental frequency.
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Figure 4.1.Simulink Diagram for Grid Connected Hybrid System with STATCOM
Figure 4.2 FFT Analysis for Load current without STATCOM
THD Analysis with STATCOM
Thus FFT Analysis shows that the voltage and current
waveforms gets smoothened with Insertion of
STATCOM across PCC in the grid. The computation
results in figure 4.4 and figure 4.5 shows that the
Current THD is about 0.86% of fundamental frequency and THD for Voltage is about 4.52% of
fundamental frequency. The FFT analysis proves that
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the voltage and current THD are in the desired limits. .
The magnitude of grid voltage and current is maintained at constant. The performance improvement can
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be analyzed with switching the STATCOM to ON and
OFF to ensure the reliability of the power system.
Figure 4.3 FFT Analysis for Load Voltage without STATCOM
Figure 4.4 FFT Analysis for Load current with STATCOM
Figure 4.5 FFT Analysis for Load Voltage with STATCOM
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5. CONCLUSION
The controlled static compensator (STATCOM) is
configured to regulate the terminal voltage with certain degree of accuracy. Herein it has been also observed that the transient stability can be increased by
maintaining the transmission voltage at midpoint. This
can be further enhanced by temporarily increasing the
voltage above the regulation reference. The
STATCOM-based control scheme for power quality
improvement in grid is connected Hybrid Energy
Conversion system and with non linear load. The
power quality issues and its consequences on the
consumer and electric utility are presented. The operation of the control system developed for the
STATCOM-BESS in MATLAB/SIMULINK for
maintaining the power quality is simulated. It has a
capability to cancel out the harmonic parts of the load
current. It maintains the source voltage and current
in-phase and support the reactive power demand for
the load at PCC in the grid system, thus it gives an
opportunity to enhance the utilization factor of
transmission line. The integrated Hybrid Energy
Conversion and STATCOM with BESS have shown
the outstanding performance. Thus the proposed
scheme in the grid connected system fulfills the power
quality norms as per the IEC standard 61400-21.This
paper has unlocked many gates for the future researchers to work further on the variable-speed variable-pitch DFIG-based wind turbine system, its voltage regulation and Efficient Utilization of Photo
Voltaic Energy into the Utility Grid.
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